1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly, to an input/output sense amplifier of a semiconductor memory device.
2. Description of the Related Art
In general, a dynamic random access memory (DRAM) includes a plurality of memory cell arrays, each divided into a plurality of sub-arrays. Each memory cell included in a memory cell sub-array is randomly accessed to store data therein and to output the data stored therein.
FIG. 1 shows a conventional semiconductor memory device. When writing data to a memory, data DI is inputted to an input buffer 18 and latched by an input latch 20. After global input/output lines GIO and /GIO are precharged by a precharge circuit 24, the latched data is applied to the global input/output lines GIO and /GIO via an input driver 22. Here, one memory cell within a memory cell array 10 is selected by a word line WL activated by a row decoder 14 and a column selection line CSL activated by a column decoder 16, and the data on the global input/output lines GIO and /GIO are written to the selected memory cell via a local input/output line LIO.
Also, when reading data written to the memory device, a specific word line WL is activated by row decoder 14. Then, the cells connected to word line WL transfer data to corresponding bit lines and bit line sense amplifiers (not shown) sense the signals on the bit lines. Data from a bit line among the plurality of bit lines is selected by column selection line CSL activated by column decoder 16, and transferred via local input/output line LIO to global input/output lines GIO and /GIO that were precharged by precharge circuit 24. The data on global input/output lines GIO and /GIO are amplified by an input/output (I/O) sense amplifier 26, and then outputted via an output buffer 28.
As described above, when writing data to the memory or reading the data from the memory device, global input/output lines GIO and /GIO are precharged. Then, data is written to or read from precharged global input/output lines GIO and /GIO. In particular, when writing data in the conventional memory device, the precharge level of global input/output lines GIO and /GIO is set to be the same as the precharge level when the data is read from the memory device.
FIG. 2 is a graph showing the change in potential over time on global input/output lines GIO and /GIO in the semiconductor memory device of FIG. 1. Writing occurs during intervals T12 and T14 and reading occurs during interval T16. Before writing or reading, global input/output lines GIO and /GIO are precharged during intervals T13 and T15. As shown in FIG. 2, the precharge levels of global input/output lines GIO and /GIO are equal to 1.45V in intervals T14 and T16.
Also, in order to reduce power consumption in memory devices, power supply voltages for such devices have decreased. As the power supply voltages have decreased, the precharge levels of the input/output lines have correspondingly decreased. However, if the precharge level is too low, a semiconductor device cannot operate normally. Such abnormal operation is more serious when reading from the memory device than when writing to the memory device. In particular, when the precharge level is low, an input/output sense amplifier does not operate normally, making it difficult to obtain the correct data from the memory device.